This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 100 01 997.8 filed in Germany on Jan. 19, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to the field of power plant technology. It concerns an integrated power plant and a method of operating such an integrated power plant.
The present invention relates to the field of power plant technology. It concerns an integrated power plant according to the preamble of claim 1 and a method of operating such an integrated power plant.
An integrated power plant of the type referred to has been disclosed by the article by G. Bauer et al.
xe2x80x9cDas Verbund-Kraftwerkxe2x80x94eine neue Variante des kombinierten Kraftwerksxe2x80x9d [The integrated power plantxe2x80x94a new variant of the combined-cycle power plant], VGB Kraftwerkstechnik 73, No. 2, pp. 120-121 (1993).
A conventional coal-fired power plant (for example a hard-coal-fired power plant) essentially comprises a coal-fired steam generator, a condensing turbine and a condensate/feedwater heater. Such an exemplary coal-fired power plant forms the right-hand part of the integrated power plant 10 shown in FIG. 1 and consists of those plant parts which are designated by the reference numerals 21 to 50 (with the exception of reference numeral 43). The thermal circuit diagram shows a water/steam circuit 49 with simple reheating (reheater 22), a three-cylinder steam turbine 26 with a high-pressure stage 27, an intermediate-pressure stage 28 and a low-pressure stage 29, and regenerative heating with a condensate heater 37 and a feedwater heater 31.
The preheated feedwater passes into the steam generator 21. The live steam generated is delivered to the high-pressure stage 27, is then reheated, and is then expanded in the stages 28 and 29. The steam turbine 26 drives a generator 30. A condenser 42 is arranged downstream of the low-pressure stage 29 of the steam turbine 26. The resulting condensate is pumped by a condensate pump 41 through the condensate heater 37 into a feedwater tank/deaerator 33. From there, a feedwater pump 32 pumps the feedwater through the feedwater heater 31 into the steam generator 21.
The coal-fired steam generator 21 receives crushed coal via a mill 24 and fresh air, which is necessary for the combustion, via a forced-draft fan 36. The resulting flue gas is cleaned after flowing through the steam generator 21 and delivered to the environment via a stack. A hot flue-gas NOx-reduction unit (high-dust DeNOx) 23 is provided as the first exhaust-gas cleaning stage. Further cooling of the flue gases is effected in an air heater 25. Provided downstream of it is an electrostatic precipitator 34. The induced-draft fan 35 then delivers the flue gases to a flue-gas desulfurization unit 39. A regenerative flue-gas heater 38 may also be arranged in between.
It is essential with regard to the invention that the flue-gas temperature of such a conventional steam generator or boiler depends on the load to a very high degree, and that, at the same time, the flue-gas NOx-reduction units, frequently used nowadays, in a high-dust circuit require a certain temperature range of about 320xc2x0 C. (280xc2x0 C.) to 400xc2x0 C. for the flue gas.
Another known type of power plant is the so-called combined-cycle power plant in which a gas-turbine set and a conventional coal-fired power plant are interconnected on the flue-gas side. In this case, the advantages of a combination of the gas-turbine cycle and the Rankine cycle are utilized by utilizing the waste heat and the residual-oxygen content of the gas-turbine exhaust gas. The gas turbine in this case is used, as it were, as a forced-draft fan for the conventionally fired steam generator. During normal operation, the gas turbine exhaust gas serves the coal firing as an oxygen carrier. If the gas turbine is shut down or has failed, the steam generator and steam turbine can continue to be operated by a back-up forced-draft fan and an adequately dimensioned steam and flue-gas air heater. Such a combined-cycle power plant is described or shown, for example, in EP-B1-0 591 163 or VGB Kraftwerkstechnik 71, No. 2, page 84 (1991).
However, on account of the type of coupling, the flue-gas-side interconnection of the gas-turbine set and the conventional coal-fired power plant in the case of the combined-cycle power plant has disadvantages, which have then led to the concept of the integrated power plant, in which the coupling between gas-turbine set with heat-recovery boiler and coal-fired power plant is restricted to the water/steam circuit. Various possibilities of this coupling between heat-recovery boiler and water/steam circuit are disclosed in the publication mentioned at the beginning.
If selective catalytic reduction (SCR), which is preferred nowadays, according to the high-dust method is now used in such an integrated power plant for the flue-gas NOx reduction of the coal-fired steam generator, care must be taken to ensure that the temperatures remain within the abovementioned temperature range of the catalyst (approx. 280xc2x0 C. or 320xc2x0 C. to 400xc2x0 C.) even at part load. To this end, the following additional measures have been disclosed hitherto:
the economizer (for heating the feedwater) arranged upstream of the catalyst contains a bypass, so that less heat is extracted from the flue gas when required by opening the bypass;
a bypass is provided for the flue gas, via which bypass the economizer is bypassed when required and the heat extraction from the flue gas can thus be reduced;
a start-up part-load heat exchanger, which relieves the load on the economizer, is used for the feedwater heating.
These are measures which are exclusively intended to keep the flue-gas temperature (at part load) upstream of the catalyst at a high level by relieving (reducing) the flue-gas-side heat absorption in the economizer.
In principle, the load on the economizer of the steam generator may be relieved by proportional heating and evaporation of feedwater in the heat-recovery boiler in those circuit variants of the integrated power plants with (at least proportional) live-steam generation. This helps to improve a part-load behavior of the conventional steam generator with regard to the flue-gas NOx-reduction unit. However, these measures must be regarded as being restricted in their potential. There is in any case no such improvement in other circuit variants.
The object of the invention is therefore to provide an integrated power plant having a flue-gas NOx-reduction unit according to the xe2x80x9chigh-dustxe2x80x9d method, which power plant has an improved part-load behavior with regard to the flue-gas NOx reduction irrespective of the respective coupling between heat-recovery boiler and water/steam circuit, and to specify a method of operating it.
The essence of the invention is to keep the flue-gas temperature upstream of the flue-gas NOx-reduction unit within the predetermined temperature range by specifically feeding some of the gas-turbine exhaust gas to the stream generator even at part load of the same.
The object is achieved by all the features of claims 1 and 9 together. The essence of the invention is to keep the flue-gas temperature upstream of the flue-gas NOx-reduction unit within the predetermined temperature range by specifically feeding some of the gas-turbine exhaust gas to the steam generator even at part load of the same.
A first development of the invention is characterized in that means for setting or controlling the branched-off portion of the exhaust gases coming from the gas-turbine set are provided, and in that the setting or control means comprise a damper arranged in the branch line and a damper arranged in the exhaust-gas duct leading from the heat-recovery boiler to a stack. As a result, it is possible in a simple manner, for various part-load cases, to in each case optimally set the proportion of the added gas-turbine exhaust gases with regard to the catalyst.
A second development of the invention is distinguished by the fact that the heat-recovery boiler contains a heating-surface system for generating and/or superheating steam for the water/steam circuit, and that the branch line branches off from the heat-recovery boiler downstream of the heating-surface system in the exhaust-gas-side direction of flow. Due to this heating-surface system, the exhaust gases of the gas turbine are cooled down (to 350xc2x0 C. to 400xc2x0 C.) to such an extent that they can be fed without further treatment directly to the steam generator. The heat-recovery boiler preferably contains a further heating-surface system for the condensate heating for the water/steam circuit, and the branch line branches off from the heat-recovery boiler upstream of the further heating-surface system.
A third development of the invention is characterized in that a heating surface, in particular in the form of an economizer or a sectional economizer, is arranged upstream of the flue-gas NOx-reduction unit in the flue-gas-side direction of flow, and in that the branched-off portion of the exhaust gases coming from the gas-turbine set is added upstream of this heating surface to the flue gases coming from the steam generator.
A fourth development of the invention is characterized in that the branched-off portion of the exhaust gases coming from the gas-turbine set is added directly upstream of the flue-gas NOx-reduction unit to the flue gases coming from the steam generator.
A fifth development of the invention is characterized in that the heat-recovery boiler for the generation of steam and/or for the superheating of steam is incorporated in the water/steam circuit.
A preferred development of the method according to the invention is distinguished by the fact that the flue gas is delivered from the steam generator to a stack by means of an induced-draft fan arranged downstream of the steam generator, and that the induced-draft fan is operated in such a way that the exhaust-gas-side pressures in the heat-recovery boiler do not change. This ensures that the output of the gas turbine is not reduced by increased pressure losses.
A further preferred development of the method according to the invention is distinguished by the fact that the flue gas is delivered from the steam generator to a stack by means of an induced-draft fan arranged downstream of the steam generator, and that the induced-draft fan is operated in such a way that the exhaust-gas-side pressures in the heat-recovery boiler are reduced and the output of the gas turbine is increased as a result.
Further embodiments follow from the dependent claims.
The invention is independent of actual parameters, functional principles, types of construction, designs and the like of steam generators and heat-recovery boilers.